Children of farmworkers in California are exposed to a unique mix of chemicals that may affect both their reproductive and neurobehavioral development. Potential exposures include the pesticide dichlorodiphenylthrichloroethane (DDT), which was recently used in Mexico where most of their farmworker parents originated; manganese (Mn), which is a key component of some widely used agricultural fungicides; and polybrominated diphenyl ether (PBDE) flame retardants, which are prevalent in California due to the state's strict flammability standards. DDT, Mn, and PBDEs are endocrine disruptors and neurotoxicants in animals, but studies in humans are limited. Age at onset of puberty in girls has declined in recent decades, possibly due to endocrine-disrupting compounds like DDT, Mn, and PBDEs, but almost no studies have examined pubertal onset in boys. These pollutants may also jeopardize children's cognitive development, particularly with exposure during the prenatal and peri-adolescent periods of peak brain growth. We propose to investigate the associations of DDT, Mn, and PBDE exposure in utero and at age 9 with puberty onset and neurodevelopment in the CHAMACOS longitudinal cohort of Mexican-American boys in the Salinas Valley, California. Boys enrolled in the CHAMACOS Study have been followed since before birth until 7 years of age. We will continue to follow these boys from age 9 to 13 years and enroll ~150 new 9 year-old-boys from the same source population, expanding the study from ~150 boys to 300. We have measured DDT and PBDE levels in prenatal maternal blood (original cohort) and will back-extrapolate prenatal levels from maternal blood collected at 9 years (new enrollees). Prenatal Mn exposure will be measured in boys' teeth. DDT, Mn, and PBDE concentrations at age 9 will be measured in boys' blood. We will assess the relationship of in utero (measured or extrapolated) and age 9 (measured) concentrations of DDT, Mn, and PBDEs with neurodevelopment (specifically general cognition, attention, memory, motor, executive function, and social cognition) and with the timing and tempo of their pubertal onset as assessed by clinical Tanner exams conducted every 9 months between age 9 and 13 years and hormone levels measured at 12 years. Thus, this proposal takes advantage of a rich dataset of Mexican-American children participating in the CHAMACOS longitudinal cohort study and proposes to expand this unique cohort and follow the children into early adolescence.